Typically multi-band and multi-mode operation technologies such as Edge, UMTS, and other 3G technologies employ a wideband antenna to support the wide operating frequency operating range. Such antennas typically consist of a connector to attach the antenna to a wireless communication device, a matching circuit on a small printed circuit assembly to match the wireless communication device to the antenna, and the actual “wire” antenna. Portions of the connector, the matching circuit, and the wire antenna are then molded in plastic to provide rigidity to the combined structure as well as electrical and mechanical protection to the matching circuit. This overmolding completely encapsulated parts of the antenna connector assembly, the printed circuit assembly and the electronic matching components, and the lower hinge part of the antenna assembly.
FIG. 1 illustrates an exemplary dual band antenna providing an overmolding in accordance with the prior art. The antenna has electrical components that make up the matching circuit. The electrical components of the matching circuit include inductors and capacitors that provide a dual band resonant matching function such that the antenna is most efficient in the 790-1020 MHz band and the 1750-2010 MHz band.
FIG. 2 illustrates the matching circuit of a conventional dual band antenna in accordance with the prior art.
As shown in FIG. 1, the matching circuit is overmolded (e.g., encapsulated with plastic) to provide structural support and protection for the antenna system. A disadvantage of such a scheme is that the overmolding significantly alters the electrical characteristics of the antenna.
FIG. 3 illustrates the difference in the measured bandwidth between an antenna assembly that has been overmolded and an antenna assembly that has not been overmolded in accordance with the prior art. As shown in FIG. 3, one of the many parameters which can be used to characterize an antenna's performance is the Voltage Standing Wave Ratio (VSWR). VSWR is the ratio of the maximum/minimum values of standing wave pattern along a transmission line to which a load is connected. VSWR value ranges from 1 (matched load) to infinity for a short or an open load. Typically, the maximum acceptable VSWR value for wireless antennas is approximately 2.0. This is approximately the same as a Return Loss of 9.5 dB. Such a return loss indicates that most of the signal from the transmitter to the antenna is being radiated (i.e., 90% radiated and 10% reflected). A VSWR value of 1.5 (return loss of 14.5 dB), which indicates that 96% of the signal is radiated and 4% reflected, is considered excellent.
As illustrated by FIG. 3, in which the VSWR was measured for the same antenna with (x) and without (o) overmolding, overmolding has the effect of changing the characteristics of the matching circuit such that the bandwidth of the 1750-2010 MHz band is decreased and the band center is shifted lower. This is due to the overmolding, which has a dielectric constant greater than that for air (i.e., approximately free space). Typically overmolding is made of plastic or rubber, but can be made of a variety of materials. The overmolding used to obtain the comparison data of FIG. 3 was Santoprene Thermoplastc Elastomer, which has a dielectric constant of 2.3, whereas air has a dielectric constant of 1.00054. For reference, the dielectric constant of a vacuum is 1.00000.
There are several reasons for the significant change in electrical characteristics of the antenna resulting from the overmolding. In practice, all passive electronic components exhibit parasitic capacitance and inductance, which are variations from the ideal capacitor and ideal inductor. It can be seen that a practical inductor is actually characterized by parasitic inductance and resistance. These are sometimes provided with the normal component specifications, or they can be measured in free space. These parasitic components can be incorporated into the circuit modeling to achieve the desired matching circuit characteristics. By conformably molding the matching circuit on the printed circuit assembly, the dielectric constant around the inductors and capacitors is changed due to, amongst other things, modification of the parasitic capacitance of the inductor as well as that of the capacitor. The result is that the overall matching is degraded from that of free space since the effective component model has changed from that of one in free space. The effects are usually more pronounced at higher frequencies.
This effect is well demonstrated in FIG. 3, where the high band return loss has shifted down in frequency and more importantly its bandwidth is reduced. The frequency shift can be dealt with by designing the matching in such a way that after overmolding and the resulting frequency shift we obtain the correct center frequency. However, the reduced bandwidth is not easy to compensate for and in wider band antennas cannot be recovered.
Another characteristic of the molding material, and in fact any molding or conformal coating compound, is the dissipation factor. The dissipation factor is a measure of the dielectric loss of a material. The dielectric loss angle of an insulating material is the angle by which the phase difference between applied voltage and resulting current in a capacitor, with the dielectric exclusively of the dielectric material, deviates from 90 degrees. The dielectric dissipation factor tan δ of an insulating material is the tangent of the loss angle d. In a perfect dielectric, the voltage wave and the current are exactly 90° out of phase. As the dielectric becomes less than 100% efficient, the current wave begins to lag the voltage in direct proportion. The amount that the current wave deviates from being 90° out of phase with the voltage is defined as the dielectric loss angle. The tangent of this angle is known as the loss tangent or dissipation factor. A low dissipation factor is important for plastic insulators in high frequency applications such as radar equipment and microwave parts; smaller values mean better dielectric materials. High dissipation factors usually translate to power being dissipated in the component rather than making it to the intended load (in this case, the antenna).
The dissipation factor of air is typically 0.0001, whereas typical overmolding materials are in the range 0.001 to 0.020. Depending on the material used, there can also be a frequency dependant effect (it may increase or decrease with frequency, or it may stay relatively constant). FIG. 4 illustrates the difference in the measured gain between an antenna assembly that has been overmolded and an antenna assembly that has not been overmolded in accordance with the prior art. Antenna gain is one way to measure the power loss in the matching network, since power that isn't radiated from the antenna itself is dissipated in the matching network and/or the source. As shown in FIG. 4 where the antenna, matching circuit, and connector are not encapsulated the gain is higher because the surrounding dielectric is air, which has a very low dissipation factor. Where the antenna, matching circuit, and connector are encapsulated the gain is lower due to the higher dissipation factor of the overmolding material.
The effects described above, which make it difficult to design overmolded antennas with wideband characteristics, have been addressed by attempting to model the effects of the overmolding on the parts, or simply using an ad hoc experimental method to adjust the ideal component values to provide the desired response. These solutions are disadvantageous in that it is extremely difficult to provide the desired antenna matching response because of the parasitic effects.